Vitamin b complex caused me to break out in acne, after taking it for a year, how long will it take for my skin to improve? I've stopped taking it.

There's no telling. Trying to figure out what's causing one to have acne is futile. Ignore all "pop" ideas and start using topical benzoyl peroxide 1-2x daily, perhaps 5%. It's not pleasant but it's your best friend. If it doesn't clear you, your physician can add prescription strength Rx. My acne remitted when I was 50; yours will too, someday. Until then, manage it like the serious medical problem that it is.
Just because. You are taking a vitamin does not take you "off the hook" for good diet. Did your skin improve when you stopped taking it?